1. Field of the Invention
The present invention relates to laser systems, and more particularly to high-power, end-pumped laser systems with solid-state gain media.
2. Description of Related Art
High power laser output is desired over a broad range of wavelengths and disciplines throughout the scientific, industrial and medical fields. Many systems have been developed to generate high-power. However, systems generating output power levels in excess of several hundred Watts become very complex. Also, some systems generate such high-powers only at the expense of beam quality.
In solid-state systems, in order to generate a higher output powers, the amount of energy used for pumping the gain medium is increased. However, many solid-state media exhibit thermal lensing or other problems causing aberrations in output beam. The pump energy can be applied from the side of the gain medium, known as a side-pumping, or from the end of the gain medium, known as the end-pumping, in most systems. Other systems create complex optics for filling a gain medium with pump energy. Side-pumping is relatively inefficient; so that the conversion of pump energy into laser output is low at relatively high pump energies. End-pumping is more efficient. However, an upper limit is quickly reached for end-pumped gain media, where rapid absorption of pump energy in a small volume within the first few millimeters of the gain media causes thermal fracture. Thermal damage to solid-state gain media can be controlled by sophisticated cooling techniques, such as is employed in so-called disk lasers. Also, thermal lensing which occurs in some solid-state gain media can be managed by including undoped endcaps. An undoped end-cap bonded on a gain medium prevents deformation at the surface of the gain medium due to the high absorption and heat generation on the surface.
Techniques have been investigated that increase the volume of the gain medium in which the pump energy is absorbed, and in which the resulting heat is generated, to prevent thermal damage and manage thermal lensing. One way to distribute heat generation within the gain medium which has been investigated includes reducing the doping concentration of the active material. At lower doping concentrations, less energy is absorbed within a given volume. See, Honea et al., “Analysis of intracavity-doubled diode-pumped Q-switched Nd:YAG laser producing more than 100W of power at 0.532 μm,” OPTICS LETTERS, Vol. 23, No. 15, Aug. 1, 1998, pages 1203-1205.
Butterworth, U.S. Pat. No. 6,898,231 B2 describes a laser based on a gain medium comprising neodymium Nd doped yttrium orthovanadate (“vanadate”), in which the pump energy is set at a wavelength which is absorbed with an efficiency substantially less than that of the peak absorption wavelengths, thereby allowing more of the pump energy to penetrate a greater volume of the gain medium before being absorbed and distributing the generated heat. In the Butterworth patent for example, the vanadate crystal was on the order of five millimeters long, and the doping concentration was relatively high at about 0.5 atomic percent. Hardman et al., “Energy-Transfer up Conversion and Thermal Lensing and High-Power End-Pumped Nd:YLF Laser Crystals,” IEEE JOURNAL OF QUANTUM ELECTRONICS, Volume 35, No. 4, April 1999, describes a longitudinally pumped laser with the pump wavelength detuned in order to increase the absorption length within the YLF host to about three millimeters. Pollnau et al., “Up Conversion-Induced Heat Generation and a Thermal Lensing in Nd:YLF and Nd:YAG,” PHYSICAL REVIEW B, Volume 58, No. 24, 15 Dec. 1998, p. 16076-16092, also describes off-peak pumping for a YLF host, while suggesting that off-peak pumping is not necessary for a YAG host because “rod fracture is not a problem.” (See, Pollnau et al., page 16077). See also, Wu et al., U.S. Pat. No. 6,347,101 B1; and Chang et al., U.S. Pat. No. 6,504,858 B2. The Wu et al., Chang et al., Butterworth, Hardman et al. and Pollnau et al. publications describe systems that use relatively low pump powers from diode lasers, on the order of 20 or 30 Watts. The resulting output powers of the lasers are therefore relatively small, and not suitable for many applications.
High power outputs have been achieved for Tm:YAG diode pumped lasers producing 2 μm wavelength outputs, and applying off-peak pumping with pump powers over 300 Watts. Tm doped media demonstrate two-for-one cross relaxation, improving pumping efficiency, so long as the pump intensity is high enough. Honea et al. report up to 115 W 2 μm wavelength outputs in this configuration, with off peak pumping. Honea et al., “115-W Tm:YAG Diode-Pumped Solid-State Laser,” IEEE JOURNAL OF QUANTUM ELECTRONICS, Vol. 33, No. 9, September 1997, pages 1592-1600.
It is desirable to provide a laser system generating high-quality, high-power outputs in a manufacturable configuration.